1. Field of the Invention
This invention relates to miniature, low-level chemical sensors, and particularly to miniature chemical vapor sensors utilizing thin-film acoustic resonators (TFR's).
2. Description of the Prior Art
There is a need to detect vapors at extremely low levels for numerous purposes. Applications include the detection of buried mines, chemical warfare agents, pollutants, proscribed substances, unexploded munitions, or food spoilage. Ideally a sensor for low level vapor detection should be miniature, durable, temperature stable, and extremely sensitive.
Acoustic transducers coated with chemical sorbates have been used as mass transducers to detect vapors. Various coatings may be used to selectively adsorb the vapor of interest. A small adsorbed mass changes the resonant frequency of the transducer. This change in resonance can then be detected electronically with high precision.
One type of acoustic transducer known as a Thin Film Resonator (TFR) is particularly advantageous for the detection of vapors. TFR's offer advantages because they are smaller and may operate at higher frequencies than other mass transducers, with less degradation from the addition of a sensitizing coating. Small size also offers other advantages such as the ability to lock multiple sensors in close thermal contact, ease of introducing the vapor with uniformity, and ease of production and packaging. Small free standing TFRs have been demonstrated as mass transducers by O'Toole et al, Analytical Chemistry, Vol. 64, No. 11 (Jun. 1, 1992).
To achieve high sensitivity in a TFR sensor, it is desirable that the device be extremely thin. This can be shown by considering the equations governing the resonant frequency of a bulk acoustic resonator. The bulk acoustic resonator is fabricated by placing thin conductive electrodes on the opposing faces of a thin piezoelectric layer. The sandwich thus formed resonates when its thickness is equal to a multiple of one half an acoustic wavelength. When material is added to one of the electrode faces--for instance, by adsorption upon a sorbent surface--the resonant path length increases and the acoustic frequency decreases. For small additions of mass, the change in resonant frequency is given by the Sauerbrey equation, ##EQU1## where .DELTA.m is the adsorbed mass per unit surface area, f.sub.0 is the initial resonant frequency, d is the thickness and .rho. the density of the resonator. If we define the mass sensitivity, Sm of the mass transducer for small changes in mass as Sm=(.DELTA.f/f.sub.0)(1/.DELTA.m), then the sensitivity is: ##EQU2## where d is the resonator thickness. Since the sensitivity is inversely related to resonator thickness, a thin film resonator is expected to have an extremely high mass sensitivity. As the frequency of operation is inversely related to the thickness, extremely thin film resonators operate at extremely high frequencies, in the gigahertz regions.
Unfortunately, extremely thin film resonators required for high frequency operation are difficult to fabricate, sensitive to stress, and very fragile. In order to resonate well, with high Q, the acoustic resonator must be acoustically isolated from its mechanical support, much as a gong must be suspended to ring clearly. This isolation prevents dampening and allows free oscillation of the resonator. In the prior art design used by O'Toole, supra, the electrodes and the piezolectric are acoustically isolated as suspended structures, with a portion of the substrate removed beneath the resonator. This leaves the thin resonator suspended much like a membrane. Suspension provides the necessary acoustic isolation, but the resulting structure is unavoidably fragile. The fragility in turn imposes a lower limit on the practical size of such TFR's and a corresponding upper limit on the practical frequency of operation.
Besides being inherently fragile, suspended TFR chemical sensors are difficult to fabricate, handle and package. Also, intrinsic compression stress of the sputtered piezoelectric films produces distortion, and thus poor reproducibility and yield.